A particular, but not exclusive, example of such a telephone system is the Second Generation Cordless Telephone (CT2), otherwise known as the Telepoint system. The common air interface specification for CT2 has been published as MPT 1375 by the Department of Trade and Industry, London 1989. In order to provide public access services, fixed site primary stations are placed at convenient locations, such as railway stations, banks and petrol stations. The primary stations are connected to the PSTN (Public Switched Telephone Network) and comprise a radio transceiver for communication with a roaming transportable secondary station which is also equipped with a transceiver. In CT2 a telephone call is initiated by the secondary station only. The radio link between the primary and secondary stations is a single frequency time division duplex (TDD) link. As is disclosed in MPT 1375 the transmission duplexing comprises a first time period in which the primary station transmits and the secondary station receives. This is followed after a predefined guard space by a second time period in which the secondary station transmits and the primary station receives. After another guard space the cycle is repeated.
If there are two or more primary stations mounted in close proximity a problem which arises is that unless their TDD framing structures are synchronized then it is quite likely that they will interfere with each other because when one primary station is transmitting, another primary station is receiving, and the close proximity of two or more non-synchronized transceivers will lead to interfering transmitted signals being picked-up by the switched-on receiver sections even though they are not tuned to the transmitted signal.
Synchronizing the TDD framing structures of the transceivers will significantly reduce and/or eliminate such interference. However such synchronization has to be fairly precise because MPT 1375 requires the amplitude of the RF envelope to ramp-up at such a rate that the envelope will be within 3 dB of the final amplitude at the start of the first bit transmitted at a rate of 72 kbits/second with a tolerance of .+-.100 ppm (parts per million) at the secondary station and .+-.50 ppm at the primary station. Although a master clock could be provided in the vicinity of each group of primary stations, this would not only add the cost of the fixedly sited equipments but also would require the provision of landline connections which is undesirable if for example primary stations are located on opposite sides of a road.